1. Field of the Invention
The present invention relates to an image forming apparatus and a copier.
2. Discussion of the Background
An electrophotographic process is typically one of image forming methods of charging a photoconductive photoreceptor in a dark place with, e.g., a corona discharge; irradiating the photoreceptor with imagewise light; forming an electrostatic latent image thereon by selectively scattering a charge on the irradiated part thereon; and developing the latent image with a toner including a colorant such as dyes and pigments and a binder such as polymers to form a visual toner image.
Image forming apparatuses using the electrophotographic process include an electrophotographic printer, etc. Recently, the image forming apparatuses are required to have high durability in addition to producing high-quality images.
A photoreceptor in the electrophotographic process repeatedly receiving mechanical and chemical influences in repeated charging, irradiating, developing, transferring and cleaning processes gradually deteriorates and wears. A worn photoreceptor causes deterioration of its chargeability and abnormal images. Therefore, longevity of the image forming apparatus using the electrophotographic process depends on longevity of the photoreceptor in many cases, and it is quite essential that a photoreceptor having good abrasion resistance is used for the image forming apparatus to have high durability.
For example, Japanese Laid-Open Patent Publications Nos. 1-205171, 7-333881, 8-15887, 8-123053 and 8-146641 describe technologies to improve abrasion resistance of a photoreceptor by forming a protection layer on the surface thereof and including an inorganic filler in a photosensitive layer thereof.
However, photoreceptors using the technologies described in Japanese Laid-Open Patent Publications Nos. 1-205171, 7-333881, 8-15887, 8-123053 and 8-146641 have good abrasion resistance, but light portion potentials thereof increase in long-term continuous repeated use and the photoreceptors have drawbacks of image quality deterioration such as image density deterioration. The protection layer on the surface of a photoreceptor can improve mechanical abrasion resistance thereof. However, when a foreign particle is adhered to a surface of a photoreceptor for some reason, the surface thereof tends to have a scratch causing an image defect. Therefore, it is difficult to make full use of a photoreceptor including a protection layer on the surface thereof in an electrophotographic process in some regards.
The foreign particle adhered to a surface of a photoreceptor includes a toner which is not cleaned. A toner adhered on a photoreceptor, which is not cleaned causes defective images. Therefore, even a highly-durable photoreceptor is considered to come to an end of its life, i.e., an image forming apparatus including the photoreceptor is considered to come to an end of its life, when producing a defective image.
Conventionally, a toner having a small particle diameter is used to realize high quality images. Although the toner having a small particle diameter can dramatically improve image quality, it is difficult to clean the toner having a small particle diameter. Therefore, the toner which is not cleaned tends to adhere on a photoreceptor and the problem mentioned above tends to occur.
In accordance with higher durability of photoreceptors, the problem mentioned above occurs due to not only the toner which is not cleaned but also paper powders, toner additives and other foreign particles because opportunities in which a paper powder caused by a paper used accumulates on photoreceptors, additives in a toner agglutinate thereon and other foreign particles adhere thereon increase.
In order to cope with the problem, removal of untransferred toner and foreign particles such as paper powders is prioritized, and e.g., it can be considered that a cut-pile shaped cleaning brush having a thicker or firmer base thread than a conventional thread-is used to improve toner removal capability and cleanability.
However, when the cleanability is strengthened more than necessary, a photoreceptor is abnormally abraded or a surface roughness thereof becomes large. Therefore, the photoreceptor cannot sufficiently be cleaned earlier, which causes image defects in many cases. For example, as mentioned above, when the cut-pile shaped cleaning brush having a thicker or firmer base thread is used, a point contact of a cross sectional edge of the thread with a photoreceptor scratches a surface thereof and causes an abnormal abrasion thereof, resulting in image defects.
Particularly, as mentioned above, when the cleanability is strengthened more than necessary in an image forming apparatus including a photoreceptor including a protection layer on its surface including an inorganic filler, the filler is easily released from the protection layer and the released filler tends to scratch a surface of the photoreceptor. Such a scratch on the protection layer in which an inorganic filler is dispersed is considered to be caused by an abrasion of the inorganic filler, which is released from the surface layer as the abrasion thereof proceeds due to long-term repeated use, with the photoreceptor when cleaned. A photoreceptor has innumerable scratches when cleaned unless cleaning conditions are adjusted because the inorganic filler typically has quite a high hardness. A toner which adheres to the scratches and cannot be removed causes defective stripe or micro-spot images.
Recently, such defective images tend to be produced more when a toner having a small particle diameter, particularly a spheric toner such as a polymerized toner, is used to produce higher quality images. It is difficult to produce high quality images without producing abnormal images and have high durability to keep producing high quality images for a long time.
Such problems also occur even in an image forming apparatus described in Japanese Laid-Open Patent Publication No. 8-314175, wherein a photoreceptor includes inorganic fine particles in its surface layer to decrease abrasion of the surface and a rubber blade and a brush are contacted with the photoreceptor to sufficiently clean the photoreceptor.
As mentioned above, a highly durable photoreceptor which is essential for forming images and, at the same time, a cleaning unit which fully takes advantage of the durability are indispensable for an image forming apparatus producing high quality images and having high durability.
However, such an image forming apparatus producing high quality images and having high durability is not available.
Japanese Patents Nos. 2619424 and 2793647 describe a brush cleaner having a loop-shaped portion which contacts a surface of a photoreceptor to improve cleanability and decrease damages of the photoreceptor due to cleaning. However, higher quality images and higher durability are desired.
Because of these reasons, a need exists for an image forming apparatus producing high quality images and having high durability.